The background of the invention relates to the synthesis of substituted ring systems. Using the process according to the invention, it is possible to provide, with few reaction steps, a novel route to .alpha.-substituted ring systems which optionally have further substituents. Cyclohexen-3-ol is an excellent example of a compound which can be prepared according to the invention. It is an important intermediate, for example for preparing Nylon.RTM. or Dralon.RTM. (from cyclohexanone and .epsilon.-caprolactam) and for the synthesis of fine chemicals.
Cyclohexanone and, by hydrogenation, cyclohexanol (of so-called KA oil which is used as a precursor for polymers such as Nylon.RTM. and Dralon.RTM.) can be obtained from cyclohexen-3-ol by catalytic isomerization, i.e., free from further by-products or subsequent products. Additionally, phenol is obtainable by catalytic dehydrogenation and cyclohexadiene by catalytic dehydration.
Hitherto, cyclohexen-3-ol was obtained by selective hydrogenation of phenol. This process comprises a large number of steps or partial steps. Thus, it is first necessary to produce phenol from benzene, usually by the so-called cumene process in which, in a Friedel-Crafts alkylation, isopropylbenzene is obtained from benzene. A disadvantage of this step is the high expense since this step cannot be carried out catalytically. Correspondingly large amounts of compounds such as iron salts, which either have to be disposed of or worked up, are formed in the Friedel-Crafts alkylation. With the aid of oxygen, this product is subsequently rearranged into phenol and acetone. As a general principle, the co-product acetone is generated in this process. The phenol which has been obtained in this manner is subjected to selective hydrogenation which is stopped at the stage of the cyclohexen-3-ol. Accordingly, this process is expensive, generates large amounts of waste products which are difficult to recycle. Additionally, the process also generates the by-product acetone, which is not always desired. A review is given, for example, by K. Weissermel and H.-J. Arpe in "Industrielle Organische Chemie", 4th Edition, 1994, VCH Verlagsgesellschaft Weinheim, pp. 375-379.
There was therefore a need to develop a process which improves the preparation of (.alpha.-substituted ring systems such as cyclohexen-3-ol from easily obtainable starting materials, with a simultaneous reduction in costs.
In principle, the olefin metathesis (a description of this reaction type is given, for example, in M. Schuster, S. Blechert, Angew. Chem. 1997, 109, 2124 and S. Armstrong, J. Chem. Soc., Perkin Trans. 1, 1998, 371), could be considered to be a feasible route for the synthesis of cyclohexen-3-ol and other .alpha.-substituted cycloolefins of the same or a greater ring size. The dienes required as starting materials are easily obtainable by the so-called telomerization reaction or by other reaction routes known per se.
By olefin metathesis of functionalized terminal dienes, it is easily possible to obtain various products catalytically by inducing a ring closure of the diene, and ethylene is obtained as a further product of value in this reaction. However, the prior art does not disclose any generally applicable specifications for carrying out certain syntheses.